Distortion correction in wave transmission systems



Sept. 19, 1933.

E. PETERSON 1,927,070

DISTORTION CORRECTION IN WAVE TRANSMISSION SYSTEMS Filed Feb, 27, 1932 AMPL/ TUOE OF THIRD ORDER MODULA T/OIV SIGN OF THIRD ORDER MODULA T ION FIG. 2

LOAD IMPEDANCE 1000 5000 w 0 2 3 o 5 E fix: I (INPUT VOL 1,405 AS PARAMETER) MID BRANCH RES/.5 mucsarms F/a 4 ET 0 so o I200 M00 2000 2400 E 3 400 INVENTOR E. PE TERSON BUM/MW A TTORNEV Patented Sept. 19, 1933 UNITE srA DIS 'IORTION CORRECTION IN WAVE TRANSMISSION SYSTEMS Eugene Peterson,

Bell Telephone Lab New York, N. Y., assignor to cratories Incorporated,

Application February 27,

- This inventionrelates to electrical transmission and particularly to the reduction of distortion incident to the transmission of electrical waves over systems employing space discharge devices as transmission elements therein, for example, vacuum tube amplifiers.

It is well known that the electric space discharge devices employed in signaling systems, because .of inherent distortion characteristics, produce in their output circuits certain modulation products in addition to the waves of the frequencies which are impressed on their input circuits. Thus, when a wave of a single frequency is impressedon the input circuit of a space discharge device the output circuit will contain a wave of the fundamental frequency and other waves of frequencies harmonically related to the fundamental. If a complex wave, that is, onecomposedof a plurality of frequen cies, is impressed on the input circuit of the space discharge device, the output circuit of the device will contain waves of frequencies harmonically related tothe frequencies of the impressed waves, waves having frequencies equal to the sum anddifference of the respectivefunda mental impressed frequenciesand various higher order modulation components.

When a space discharge device is utilized in a signaling system for a certain purpose, for example,amplification, it is desirable from the standpoint of' quality'in transmission that, the effects of allmodulation products produced in the output circuit of the device be "minimized. Asfthe effects on quality of transmissionof the modulation products of orders higher than the third. are usually negligible compared to the effects of the second andthird order modulation products, if the third and second'order products present areeliminated or reduced to a negligible volume by suitable means, the system for all practical purposes is made distortionless.

In a long, non-loaded signaltransmission line having small phase distortion, comprising N identical, equally spaced repeaters, the thirdorder modulation output is made up of the arithmetical sumoi the modulation in the N repeaters. The thirdv order modulation output at the end of the line is, therefore, N timesas large, with respect to the fundamental, as it is in asingle one'of the repeaters. .The same holds true for second or der and forother orders of modulation. This factor obviously imposes stringent modulation requirements in the design of the individual .repeaters,- which requirements are more and more difficult to meet as in the line increases.

rss PATENT oFFics the number of repeaters An object of the present invention is to control modulation in signal wa ve transmission systems.

A related and more specific object is to reduce the effects of modulation, including both odd and even orders,in a signaling system comprising a plurality of vacuum tube transmission devices, for example, vacuum tube repeaters.

In accordance-withthe invention, modulation distortion in a signaling system comprising a plurality of vacuum tube transmission devices, is minimized by so selecting the vacuumtubes therein and adjusting istics as to pro their circuit characterduce randomness in sign of both third order and even order modulation outputs of the individual transmission devices The invention will be better understood from the following detailed description thereof: when read in connection with the accompanying drawing in which:

Fig. 1 shows diagrammatically a long signal ing system comprising a plurality of vacuum tube repeaters connected in tandem and embodying'the invention;

tube repeater which is a the invention; and.

-Figs. 2 and 4 show characteristic curves of typical vacuum tube amplifiers illustrating the invention.

The U. S. pat

January 26, 1926 discloses a been found practical f ent to Nyquist No. 1,570,770, issued method which has or reducing even order modulation in a system comprising a plurality of vacuum tube repeaters connected in tandem.

This is accomplished by ers in the system oppos order distortion compon poling successive repeatitely. Because theeven ents have a phase angle which; is an even multiple of the phase of the fundamentals, the even their original phase when the fundamentals are reversed in phase. Thus, if'the polarity of a repeater is reverse input or output indicated by the crossed leads bet tube repeaters R1, R2, R3, etc. in

cessive vacuum d, for example, byreversing the leads to the repeater-tubes, as-

ween the sucthe system of Fig. 1, the phase of the even order modulation products with respect mental changes by,,180 degrees. poling all of the repeaters R1,

to the funda- By properly R2, RN, half may be'made to have even order modulation of onesign and the other of the opposite sign, so

half evenorder modulation thata random distribuder components retain Heretofore, in connection with a study of modulation in vacuum tubes, due to the variation of the amplification factor, and of the internal plate resistance, R0, it has been found that the characteristics of certain types of vacuum tubes are such that the sign of the third order modulation changes as the load impedance is varied from small to large values due to the summation of ,u. and R0 modulations of opposite sign. In some tubes, the point of zero third order modu-- lation was found to be not far from the matching resistance of the tube. The variation in' the amplitude of third order modulation for the two space discharge devices of this general type in a balanced vacuum tube amplifier, with load impedance is shown in curves at and b, respectively, in Fig. 2. The summation of the currents represented by these two curves is shown by a curve 0 which passes through the zero axis at a load resistance of approximately 1500 ohms indicating that the third order'products of the two tubes are of opposite phases. The curves of Fig. 2 were taken from Landeen Patent 1,722,099, issued July 23, 1929. i

In a batch of tubes having the above mentioned characteristics, 'manufacturing variations inevitably occur so that it may not always be feasible to set the operating point of each tube or pair of tubes in a balanced amplifier for zero third order modulation output. However, an operating point can be readily chosen for all of the tubes in the batch such that the distribution of third order modulation among them is approximately random in sign and amplitude. If tubes of the type described are used in the repeaters R1, R2, R3, RN, in the system of Fig. 1 and the load impedances thereof are adjusted properly so that the sign of the third order 'modulation products of approximately half of them is opposite to that of the other half, the third order modulation output of the system may be reduced by where N is the number of repeaters, compared to that obtained'by the use of other tubes having the same absolute value of modulation but lacking the randomness in sign. If then the even order modulation output of the system is reduced to a negligible value by poling the successive repeaters oppositely so that the even order modulation produced by half of the repeaters is balanced by the even order modulation of opposite sign produced by the other half of the repeaters, the modulation output of the system is greatly reduced.

The invention is not limited to the use of tubes of the characteristics described above for obtaining the randomness in sign and amplitude of the third order modulation products, which impose certain limitations on the load impedance, which insome cases may be undesirable. Other methods -of obtaining the randomness in sign and amplitude of the third order modulation products which are independent of the load impedances and which apply to all types of tubes may be used. For example, the method used may be that shown inthe copending application of J. G. Kreer, Jr., Serial No. 595,502, filed February 2'7, 1932 which discloses that the sign of the third order modulation produced in any three-electrode space discharge device may be made to change from positive to negative sign by merely changing the value of an impedance element or elements in the external circuits connecting the electrodes (not the load circuit) of the device.

Fig. 3 shows a preferred embodiment of this method applied to a balanced Vacuum tube amplifier circuit, taken from the Kreer application. As described in detail in the Kreer application, by suitable choice of the value of the resistance element 1 in the common branch of the output circuit of the balanced amplifying tubes 2 and 3 the sign and amplitude of the third order modulation products transmitted to the load circuit L may be controlled in the desired manner.

The curves d, e and f in Fig. 4, taken from the Kreer application, show the variation in amplitude and sign of third order modulation with variation in the mid-branch resistance for three different input voltages in a balanced vacuum tube amplifier circuit corresponding to that of Fig. 3 employing Western Electric Company No. 104D vacuum tubes. Similar curves may be obtained by using any other types of tubes.

In the operation of the system of Fig. 3, the impedance of the load circuit L may be set at the optimum value from the standpoint of maximum power output or other considerations and then the value of the resistance 1 in the mid-branch may be chosen so as to give the desired magnitude and sign to the third order modulation products transmitted to the load circuit. It is apparent that if amplifier circuits such as shown in Fig. 3 are used in the repeaters R1, R2, RN in the system of Fig.1 a simple adjustment of the output mid-branch resistance in the several repeaters will give the desiredrandomness in sign and amplitude of the third order modula tion products.

Although this invention has been described in connection with particular types of vacuum tubes, and with specific types of repeater circuits, it is not limited to these types of tubes or circuits but is capable of broad application and is to be limited only by the scope of the appended claims.

What is claimed is:

1. In a wave transmission system comprising a plurality of wave transmission devices including one or more electric space discharge tubes, the method of reducing the effects of modulation due to the distortion characteristics of said tubes which consists in so selecting the constants of the circuits associated with said spacedischarge tubes that the odd order modulation outputs of certain of said devices are of opposite sign and of approximately the same order of magnitude and poling said devices so that at least a portion of ,the even order modulation products of certain of said tubes is effectively.

neutralized by those'of other tubes.

2. In a signal wave transmission system comprising a plurality of wave transmission devices including one or more electric space discharge tubes, the method of reducing the effect of modulation components of the transmitted signals produced by said tubes, which consists in selecting for the tubes in certain of said devices ones of the types in which the third order modulation changes sign for difierent values of load impedance, and adjusting the load impedances for the various tubes of that type so that the third order modulation of certain tubes are effectively neutralized by the third order modulation of others.

3. A high quality signal wave transmission system comprising a plurality of wave changing devices certain of which contain one or more electric space discharge tubes, certain of said tubes neutralized by those of others.

4. A high quality signaling system comprising a plurality of vacuum tube repeaters connected in tandem, certain vacuum tubes in a plurality of said repeaters being of the type in which the sign and magnitude of the third order modulation output depends-on the value of the associated load impedance, and the values of the load impedances of said certain tubes being such as to produce such distribution of signs and magnitudes in their third order modulation outputs that the third order modulation of certain repeaters is effectively neutralized by that of others and means for poling said repeaters so that at least a portion of the second order modulation products of certain repeaters is effectively neutralized by those of other repeaters. 1

5. A high quality signal wave transmission system comprising a plurality of wave repeating devices connected in tandem certain of which devices include one or more space discharge tube amplifying elements comprising input and output circuits, the impedance of at least one of said, circuits for at least one of saidelements being chosen to make the third order modulation produced thereby of such sign and amplitude as to neutralize effectively the third order modulation produced by other elements.

6. A signal wave transmission system comprising a signal line and wave repeating devices at various points'along said line, each device comprising two space discharge devices having portions of their input and output circuits in common and a resistance element in the common portion of the output circuits of the two tubes for each device, the values of the, resistance elements being chosen so as to make the third order modulation outputs of a portion of said devices efiectively opposite in sign to those of the remaining portion and of such magnitude relative to the magnitudes of the third order modulation outputs of said remaining portion as to reduce the third order modulation at the output, of said line to a negligible amount.

7; A signal wave transmission system comprising a'signal line and wave repeating devices at various points along said line, each of said repeating devices comprising two space discharge amplifying tubes having their input and output circuits connected in push-pull relation, a resistance element in the common portion of the output circuits of the two tubes in each repeating device, the value of the resistance elements be- 100 ing chosen of such value to make the third order modulation outputs of a portion of said repeating devices effectively opposite in sign to those of the remaining portion and of such magnitude relative to the magnitudes of the third order 105 modulation outputs of said remaining portion as 

